1. Field of the Invention
The present invention relates generally to the field of electrical discharge machining more particularly to apparatus for abrasively forming electrical discharge machining electrodes.
2. Description of the Prior Art
Prior to the present invention a method and apparatus for forming electrical discharge machining electrodes were disclosed in U.S. Pat. Nos. 3,663,785 entitled Method of Erosively Shaping a Master Die and Re. 27,588 entitled Apparatus for Shaping Electrodes, respectively. The method and the apparatus for its implementation involved the use of an abrasive die and an electrode workpiece mounted in working alignment with one another. The die and workpiece are moved relative to one another in a horizontal oscillatory rotary motion as they are brought together in the vertical direction such that the workpiece is abrasively formed into the same shape and contour as the die member.
The relative vertical motion of the die and workpiece in the prior art method and apparatus was cyclic to permit removal of abraded particles by flushing with a solvent. The infeed and outfeed portions of the cycle were originally timer controlled at a constant speed with each infeed portion being slightly longer in time than the outfeed portion. A pressure sensor attached to the vertical drive mechanism was used to reverse the drive a a preselected pressure level. The problem encountered with the prior art method and apparatus was that as the abrading process progressed and the contact surface area of the die and workpiece increased the pressure at the interface would vary. During the initial stages of the process when the interface area is small a given pressure would be appropriate, however, as the work progressed and the interface area increased that same pressure level would be too low for efficient abrading. Operation at a constant infeed/outfeed rate thus resulted in damage to both abrading dies and electrodes. If the rate were reduced to thereby relieve the interface pressure the outfeed portion of the cycle was too slow to maintain efficiency.
One partial solution to the pressure problem was to use a first relatively high rate on the first portion of the infeed, a second relatively lower rate during the portion of the infeed while abrasion was being accomplished and to return to the relatively higher rate for the outfeed portion of the cycle. Switching between the two rates was performed as a function of time. While beneficial to some degree this approach still failed to solve the pressure problem since as the die-workpiece interface area increased the operation of hydraulic valves would become inconsistent, the infeed distance would increase and the outdistance would decrease. It was therefore necessary for a skilled operator to constantly monitor the cycle timer and to make continuous adjustments preferably in anticipation of excessive pressure build-up.